The Tricky Little Secret To Better Gas Mileage

by Don Elliott on September 4, 2012

The U. S. Transportation Secretary Ray LaHood announced recently that Corporate Average Fuel Economy (CAFÉ) standards would gradually increase to a proposed 54.5 miles per gallon by 2025. The newly announced CAFÉ requirements are a significant increase in the previous target of 35.5 mpg by 2016 set by the Department of Transportation.

In announcing the requirements, the government anticipates that the new rules will stimulate investment in new technologies, encourage innovation and create jobs. The car manufacturers do not yet have any idea how they are going to comply with the new requirements. However, one way to milk more horsepower out of smaller combustion engines and therefore improve gas efficiency is by using turbochargers.

We have seen an increase in the number of turbocharged engines from the European car manufacturers. The 2012 Volkswagen Passat base model car offers a 2.5L 5-cyl engine that puts out 170 horsepower, 177 lb-ft of torque and 22 city/35 highway/25 combined mpg with an automatic transmission.

The 2.0L 4cyl diesel turbo kicks out 140 horsepower and 236 lb-ft of torque yet achieves 30/40/34 mpg, a nearly 33% improvement in the fuel economy of the comparably equipped non-turbo gasoline version. It would be naïve to assume that VW and consumers will overlook this obvious improvement in used car value generated by the turbo diesel powerplant.

From the domestic manufacturers, turbochargers are also becoming more prevalent but without the emphasis on the word “turbocharger”. Ford calls their secret for better gas mileage “Ecoboost”. The 2013 Ford Taurus offers an optional 2.0L 4-cyl turbo versus the standard 3.5L V-6 with an expected 20% improvement in fuel economy

So what is the secret behind turbocharged engines and why haven’t we seen them introduced more aggressively? Within non-turbocharged engines, oxygen is the limiting factor regarding power output. A gasoline engine requires a 14:1 air-to-fuel ratio to operate efficiently. Using normal air pressure, the only way to get more power is to use a bigger engine, much to the pleasure of automotive repair shops. To increase the amount of oxygen in the combustion chamber, an air pump called a turbocharger is used to condense the incoming air, increasing the power output from a smaller engine. The turbocharger, powered by exhaust gases, kicks in only when more power is needed, giving the engine an extra boost.

Cost has always been a barrier to more turbocharger usage. Depending on how the turbocharger is packaged with other equipment, the cost can be $1500 to $2000 more than a similarly equipped non-turbocharged model. In addition, the cost for auto repair  to fix turbocharged engines has caused some buyers to avoid them even though they offer more power and significant fuel savings.

As is typical with many of the fuel saving initiatives associated with the higher CAFÉ standards, the cost to consumers for more fuel-efficient vehicles is often overlooked. Ray LaHood is quick to point out the fuel saving achieved by the higher average fleet numbers. However, the cost of the more expensive fuel saving mechanisms like turbochargers will go right back to the consumer.

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